Yeah. That sounds right to me. For first order Ambisonic mastering, you'd need to use four channel plugins that treated each channel equally (if you are processing the B format files before decoding that is). I'm going to approach my new Ambisonic mixes from the perspective of trying to get everything right in the mix in order to minimize mastering complexity and requirements. That's not much of a jump for me though because I've always tried to work that way anyway.

Yes. I asked Bruce Wiggins. He said that as long as you are careful to use sidechains/control parameters so that all 4 channels are affected equally when you put an effect on something, mastering will work fine.